Hydraulic power brake boosters have been proposed as a replacement for pneumatic power brake boosters because of a reduction in available space under the hoods of some vehicles. In an effort to provide a uniform braking force corresponding to an operator input force, a hydraulic ratio changer as disclosed in U.S. Pat. No. 3,831,491 was introduced into such brake boosters. Unfortunately, in these hydraulic ratio changers, the change in movement of the input pedal varies with the change in volume of a hydraulic fluid in the control chamber. In an effort to compensate for volumetric change of the fluid in the control chamber, fluid communication with a reservoir is established when the input force from the operator terminates. Unfortunately, such compensation is dependent upon the position of an actuation member and not the pressure producing piston.
The hydraulic brake booster in U.S. Pat. No. 3,838,629 discloses a manually operated ratio changer having a plunger which pressurizes fluid in a closed chamber whenever an input force is applied to the boost operating mechanism. The pressurized fluid is communicated to a spool valve which in turn controls the communication of hydraulic fluid under pressure. The hydraulic fluid under pressure moves a piston in a master cylinder and provides an energizing force for operating the wheel brakes in a vehicle.